But a doctor named as a co-defendant in one suit — for allegedly taking kickbacks to prescribe the drug extensively at nursing homes — was never pursued.

Last year, Alpharma paid $42.5 million to settle federal allegations that it paid kickbacks to doctors to prescribe its painkiller Kadian.

“Health-care decisions must be based solely upon what is best for the individual patient and not on which pharmaceutical company is paying the doctor the biggest kickback,” Rod Rosenstein, U.S. attorney for the District of Maryland, said in announcing the settlement.

But the doctors, accused of trading prescriptions for paid speaking gigs, faced no consequences.

At least 15 drug and medical- device companies have paid $6.5 billion since 2008 to settle accusations of marketing fraud or kickbacks. However, none of the more than 75 doctors named as participants were sanctioned, despite allegations of fraud or of conduct that put patients at risk, a review by ProPublica found.

Behavior overlooked

Reporters reviewed hundreds of pages of court records and interviewed current and former federal prosecutors, state medical board officials, attorneys for whistle-blowers and, when possible, the doctors. For each doctor identified in a suit, ProPublica checked for state medical board discipline, penalties from the Medicare program and federal criminal charges.

In many of the cases, it appears that not even a cursory investigation was done to see whether the physicians had behaved inappropriately.

“Doctors have kind of gone under the radar,” said Tavy Deming, a Philadelphia lawyer who represents drug company whistle-blowers.

Amid concerns about the influence of drug-company money on medicine, whistle-blower lawsuits have emerged as a headline-grabbing tool for holding manufacturers accountable.

Yet, despite their power to secure large settlements from drugmakers, the suits have failed to resolve the culpability of physicians. Doctors often are not named as defendants, even though descriptions of their alleged misconduct are used to bolster the suits. And even when settling, many companies, including Alpharma, continue to deny the allegations.

After cases are resolved, the internal company documents used as evidence remain confidential, preventing exploration of the physicians’ behavior.

Lawyers not interested

Medical boards, which normally pursue tips or complaints of wrongdoing, do not routinely scan for such cases. Justice Department lawyers, wary of spending more time and effort on a case, say they usually are not interested in going after lesser players.

Tony West, the assistant attorney general who oversees civil litigation nationwide for the Justice Department, declined through a spokeswoman to discuss the issue. In announcing settlements with the drug companies, however, West has said that kickbacks undermine doctors’ credibility. Medical decisions, he said in one news release, should be “guided by a patient’s needs, not tainted by illegal incentives or fraud.”

For Justice Department lawyers, big drug companies make attractive targets for whistle-blower lawsuits. They are flush with profits and determined to avoid crippling legal defeats. Their bureaucratic sprawl often leaves a trail of incriminating e-mail and memos.

Doctors, on the other hand, make particularly unattractive targets. Fearful of losing their licenses — and perhaps going to prison — they will devote every penny to their legal defenses. And juries like to think the best of physicians.

“It’s a scorched-earth battle (for a doctor),” said Michael Loucks, a former federal prosecutor in Massachusetts who led some of the nation’s biggest health care fraud investigations. “If he’s convicted, it’s not only a federal prison sentence, but he loses his license.”

Rules governing doctors are less strict than those for drug companies. They are permitted to talk about unapproved uses of drugs or prescribe them when they believe a patient will benefit. To secure a conviction, prosecutors must show that doctors knowingly traded their prescription pads for money or perks.

Another factor weighing against prosecution is burnout. After spending years taking on a drug company, many government lawyers are loath to tack on more time for a relatively minor victory.

In Colorado, an ethics crackdown by two major academic institutions is slowing the controversial practice, The Denver Post reported recently. Since 2009, drug companies have made nearly $13.4 million in payments to doctors in the state, but an ethics board at the University of Colorado School of Medicine and National Jewish Health have been scrutinizing such payments and disallowing more contracts between doctors and drug companies to avoid undue influence.

The owners of Boulder’s Sterling University Peaks apartments, who this summer were cited for illegally subdividing 92 bedrooms in the complex, have reached an agreement to settle the case for $410,000, the city announced Thursday.